1 4ohm cab or 2 8 ohm cabs

If I get a power amp that is 1000w bridged mono into 4 ohms, is there any advantage or disadvantage in percieved volume or tone running 1 strong 4 ohm cab vs 2 good 8 ohm cabs? (Other than the hassle of lugging around 2 cabs)

Two cabinets spread apart on each side of the stage give a better stage presense even at lower volumes and better audience penetration. And for small gigs you only have to carry one. If you have a four ohm speaker you cant add one more speaker with most amps unless its a stereo amp. So two eight ohm speakers is better.

Two cabs is better than one. The perceived loudness is greater because of the increased cone area pushing more air. Also, two cabs together is better than spread over the stage, due to an acoustic phenomonon called acoustical coupling, in which multiple speakers have the effect of boosting each other. Also two cabs in a vertical configuration will throw sound further. This is why large PA rigs have the speaker cabs mounted on top of each other, sometimes 20 cabs high for stadiums.

I bass-ically agree with the other posters here. I take it your amp must be a stereo amp in order to run it bridged. Sometimes such amps can put out far more power while bridged into 4 ohms than running stereo into two 8-ohm cabinets. Here's why (it's a generalization/simplification and ignores other factors, but I want to explain it):

Power = Voltage^2/Resistance _____ (^ means exponent)

Voltage out of one side of a stereo amp is, say, Va. The power out of each side into an 8-ohm load is then Pa = Va^2/8. Two such channels gives a total power output of P_stereo = 2Pa = Va^2/4.

Now, when you bridge an amp, you basically get double the voltage capability, so Vb = 2Va. Power when bridged into 4 ohms is then P_bridged = Vb^2/4 = (2Va)^2/4 = Va^2.

You see in this example that P_bridged = Va^2, which is four times the power of P_stereo = Va^2/4.

So... I recommend comparing the output specs of stereo at 8 ohms per side vs. bridged at 4 ohms. You might be surprised. One example I have handy is the Carvin R1000 head: 2 x 225 watts at 8 ohms (total power = 450 watts), but 1000 watts bridged into 4 ohms. Impressive, huh? But 1000 watts is only 3.5 dB louder than 450 watts. (No wonder California is running out of electricity!)

Getting messy, isn't it? If you bought two 8 ohm speakers that are 3-4 dB more efficient than the one 4-ohm, then it wouldn't make much difference. On the other hand (to make it clearer than mud!), the radiation pattern of two cabinets is different than just a single cabinet, tends to improve directionality, and will therefore affect the perceived loudness in many cases (see Marty's and bassdude's comments, which I agree with).

Now that I've totally confused the issue... it really depends on the qualities and efficiencies of the speakers you are considering - as well as the stereo vs. bridged power of the amp. Another thing is that the amp will probably run hotter bridged into 4 ohms than in stereo @8 ohms. I think it could work either way, but you have to think about how much gear you want to lug around, etc. If the speakers were identical (except for impedance), the bridged configuration might be *slightly* (perhaps 1 dB) louder than two speakers in stereo.

Sorry, not an easy answer, but I hope this helps you understand the tradeoffs. I welcome any rebuttals or corrections. If you have particulars on the amp and cabinets you have in mind, it might be easier to figure this out.